1. Field of Invention
The present invention relates generally to semiconductor processing equipment. More particularly, the present invention relates to an illuminator layout of a projection tool which enables both dense and isolated patterns on reticles to be precisely projected onto a wafer surface during a lithographic process.
2. Description of the Related Art
For precision instruments such as photolithography machines which are used in semiconductor processing, factors which affect the performance, e.g., accuracy, of the precision instrument generally must be dealt with and, insofar as possible, eliminated. When the performance of a precision instrument is adversely affected, products formed using the precision instrument may be improperly formed and, hence, function improperly. For instance, a photolithography machine with an illuminator which does not allow circuit patterns or features associated with a reticle to be precisely projected onto a semiconductor wafer surface may result in the formation of integrated circuits or semiconductor chips which do function as expected.
FIG. 1 is a diagrammatic representation of a photolithography or exposure apparatus. An exposure apparatus 100 includes a reticle 104 which effectively serves as a mask or a negative for a wafer 108. Patterns, e.g., patterns formed using a thin metal layer or layers, which are resident on reticle 104 are projected as images onto wafer 108 when reticle 104 is positioned over wafer 108 in a desired position. An illuminator 112 is used to provide a broad beam of light to reticle 104. In other words, illuminator 112 distributes light. Portions of a light beam, for example, may be absorbed by reticle 104 while other portions pass through reticle 104 and are focused onto wafer 108 through lens assembly 116.
Wafer scanning stages (not shown) are generally used hold and to position wafer 108 such that portions of wafer 108 may be exposed as appropriate during masking process or an etching process. Reticle scanning stages (not shown) are generally used to hold reticle 104, and to position reticle 104 for exposure over wafer 108.
Illuminator 112 includes an illumination source 120 which provides a beam of light or a relatively broad beam of electrons. The beam provided by illumination source 120 illuminates illuminator aperture 124 which provides poles or areas through which the beam may pass. As will be discussed below, the pattern of poles provided by illuminator aperture 124 is typically dependent upon an anticipated type of patterning on reticle 104. Once a beam, or portions of the beam, passes through illuminator aperture 124, the beam is condensed by a condenser lens 128. Condenser lens 128 delivers the beams passing through illuminator aperture 124 to reticle 104 at a desired angle of incidence.
Reticle 104 may be patterned with an isolated geometry, a dense geometry, or a varied geometry. The type of patterning on reticle 104 is typically dependent upon a desired integrated circuit design to be patterned on wafer 108. When reticle 104 has a varied geometry, reticle 104 may include areas which are sparsely populated and areas which are densely populated. FIG. 2a is a diagrammatic representation of a reticle with an isolated pattern geometry, i.e., a reticle which is relatively sparsely populated. A reticle 200 includes patterned features or contacts 204 which may have at least one dimension ‘d1’ 208 that is a critical dimension. As will be appreciated by those skilled in the art, contacts 204 are generally open segments or print holes in reticle 200.
Typically, dimension ‘d1’ 208 is in the range of approximately one micron or less. More generally, the critical dimensions including dimension ‘d1’ 208 are in a range from approximately a fraction of an illumination wavelength to approximately a relatively low multiple of the illumination wavelength. When reticle 200 is considered to have an isolated geometry, then adjacent contacts 204 are typically spaced at distances of approximately a few times dimension ‘d1’ 208, or a relatively low multiple of dimension ‘d1’ 208. As shown, contact 204a is spaced apart from contact 204b by a distance ‘d2’ 212 which is generally substantially more than the distance associated with dimension ‘d1’ 208.
FIG. 2b is a diagrammatic representation of a reticle with a dense pattern geometry. A reticle 220 includes features or contacts 224 which have at least one dimension ‘d1’ 228 that is defined as a critical dimension. When reticle 220 is densely patterned, contacts 224 are typically spaced apart such that a distance ‘d2’ 232 between adjacent contacts 224a, 224b is less than or approximately equal to the critical dimension, e.g., dimension ‘d1’ 228.
The configuration of an illuminator aperture that is used in an illuminator which provides a beam, e.g., a beam of light, to a reticle is generally dependent upon the pattern of features or contacts on the reticle. In other words, an illuminator aperture is typically chosen based upon the requirements of a reticle which is to be used with the illuminator aperture. The layout of an illuminator aperture effectively defines the directions at which features on a reticle are illuminated. In addition, the layout or configuration of an illuminator aperture also defines the direction or directions in which light scatters from a reticle.
Typically, the configuration of an illuminator aperture that is to be used with a reticle which has an isolated or sparse pattern geometry varies from the configuration of an illuminator aperture that is to be used with a reticle which has a dense pattern geometry. Since the illuminator aperture serves as an attenuated phase shift mask, different illumination requirements are associated with the patterning of isolated and dense geometries. When a reticle has an isolated pattern geometry, a small sigma, on-axis illuminator aperture is used to meet illumination requirements for patterning isolated pattern images onto a wafer. Alternatively, an off-axis illuminator aperture is used to meet illumination requirements for patterning dense pattern images onto a wafer.
With reference to FIG. 3, a small sigma, on-axis illuminator aperture will be described. An illuminator aperture 300 includes a pole 304, e.g., an opening, that is positioned substantially in the center to illuminator aperture 300. Pole 304 is arranged to allow a beam such as a beam of light to pass therethrough to a reticle (not shown). Illuminator aperture 300 is configured to substantially optimize the patterning of isolated features onto a wafer (not shown). While the configuration of illuminator aperture 300 is effective for use in accurately patterning isolated features, the configuration of illuminator aperture 300 is generally relatively poor with respect to the accurate patterning of dense features.
As previously mentioned, when dense features are to be patterned, an off-axis illuminator aperture is typically used. FIGS. 4a and 4b are diagrammatic representations of off-axis illuminator apertures with substantially circular poles. A first off-axis illuminator aperture 400 with substantially circular poles 404 is arranged with four poles 404 in a square pattern, as shown in FIG. 4a. The arrangement of poles 404 generally enables precise patterning of dense features. However, the arrangement of poles 404 does not allow for the precise patterning of isolated features. Poles 414 of illuminator aperture 410, as shown in FIG. 4b, are positioned in a diamond pattern. Like poles 404 of illuminator aperture 400, the positioning of poles 414 of illuminator aperture 410 is substantially optimized for the patterning of dense features. When the positioning of poles 414 is substantially optimized for the patterning of dense features, illuminator aperture 410 does not allow for the accurate patterning of isolated features.
In lieu of having substantially circular poles, an off-axis illuminator aperture may have poles of other shapes. By way of example, poles may have substantially triangular shaped poles. FIGS. 5a and 5b are diagrammatic representations of off-axis illuminator apertures which have substantially triangular shaped poles. An illuminator aperture 500 includes substantially triangular poles 504 which are arranged in a square pattern, as shown in FIG. 5a. Substantially triangular poles 514 which are included on an illuminator aperture 510 of FIG. 5b are arranged in a diamond pattern. While both the square pattern and the diamond pattern of poles 504 and poles 514, respectively, are effective for optimizing the patterning of isolated features, neither pattern allows for the precise patterning of dense features.
When an illuminator aperture allows isolated features to be accurately patterned, the illuminator aperture patterns dense features relatively poorly. That is, when an illuminator aperture provides relatively good dimensional control of isolated feature images on a wafer, the illuminator aperture generally does not provide good dimensional control for dense feature images on a wafer. Similarly, when an illuminator aperture allows dense features to be accurately patterned, the illuminator aperture patterns isolated features relatively poorly.
Often, semiconductor wafers require areas which require isolated patterning and areas which require dense patterning. In other words, many wafers have areas which will have isolated feature images and areas which have dense feature images. Reticles that are used to pattern both isolated feature images and dense feature images on a wafer will also portions which have isolated features and portions which have dense features. When reticles include both isolated features and dense features, then the use of an illuminator aperture which is good for patterning the isolated features is not as good for patterning the dense features. Alternatively, the use of an illuminator aperture which is good for patterning the dense features is not as good for patterning the isolated features. As such, it is generally necessary to sacrifice the precise dimensional control of some feature images for the precise dimensional control of other feature images.
Sacrificing the dimensional control or the accuracy with which feature images, i.e., either isolated feature images or dense feature images, are patterned onto a wafer may cause the quality of semiconductor chips formed from the wafer to suffer. As such, when a wafer has both an isolated pattern geometry and a dense pattern geometry, the choice of either a small sigma, on-axis illuminator aperture or an off-axis illuminator aperture to use in patterning the wafer will result in the sacrifice of the accuracy with which either dense feature images or isolated feature images, respectively, are patterned onto the wafer. When some features on a wafer are inaccurately formed, the functionality, e.g., the performance, of semiconductor chips included on the wafer may be unacceptable.
Therefore, what is needed is a system and a method which enables both isolated pattern geometries and dense pattern geometries to be relatively accurately formed on a wafer. More specifically, what is desired is an illuminator aperture which enables good dimensional control of both isolated pattern images and dense pattern images formed on a wafer.